The present invention relates to photoresists which have increased light-sensitivity and are suitable for forming relief structures of highly heat-resistant polymers.
Photoresists for forming relief structures of highly heat-resistant polymers are extensively used in the manufacture of electrical and electronic components, of chemical milling resists and electroplating resists or of printing molds. Photoresists which are highly suitable for these purposes have been described in, for example, German Pat. No. 2,308,830, corresponding to U.S. Pat. No. 3,957,512. These photoresists contain radiation-sensitive polymeric precursors. They are applied to a substrate in the form of a layer or sheet, and the layer or sheet is dried and then irradiated through a negative original. As a result, cross-linking takes place in the irradiated areas and drastically reduces the solubility of the material applied there. The non-irradiated parts of the layer or sheet are then removed in a suitable manner by, e.g., dissolving them out or stripping them off. The remaining relief structures can then be heat-treated to give highly heat-resistant polymers which can withstand temperatures of 250.degree.-400.degree. without adverse effect on edge definition or resolution.
In German Patent No. 2,308,830/U.S. Pat. No. 3,957,512, the soluble polymeric precursors (prepolymers) are polyaddition or polycondensation products of (a) carbocyclic or heterocyclic compounds having two functional groups capable of polyaddition of polycondensation reactions and having radiation-reactive radicals which are bonded through carboxylic ester linkages in the ortho- or peri-position relative to the functional groups (these compounds are referred to below as ring compounds I) and (b) such diamines, diisocyanates, bis-acid chlorides or dicarboxylic acids which can react with these functional groups and which contain at least one cyclic structural element (these (b) compounds are referred to below as ring compounds II).
German Patent No. 2,308,830/U.S. Pat. No. 3,957,512 lists a number of radiation-reactive radicals which are responsible for the crosslinking of the photoresist on irradiation. It has been found, however, that to obtain adequate crosslinking of these photoresists on irradiation with a conventional source of UV rays, irradiation times of about 5 minutes, usually even 8 to 15 minutes, are required. Moreover, these times cannot be significantly shortened by means of conventional photosensitizers.
In German Patent Of Addition No. 2,437,348, corresponding to U.S. Pat. No. Re. 30,186, the photoresists of German Patent No. 2,308,830/U.S. Pat. No. 3,957,512 are modified by incorporating into the ring compounds I, as radiation-reactive groups, oxyalkyl acrylate or oxyalkyl methacrylate groups. This measure shortens the average required irradiation times by about 3 minutes. Nevertheless, even this shorter irradiation period is still too long if photoresists of this type are to be used in the mass production of electronic components. European Patent A No. 47,184 described how the light-sensitivity of photoresists which contain soluble radiation-reactive polyamide-ester resins can be increased by adding, at the same time, radiation-sensitive polymerizable polyfunctional acrylate compounds and aromatic halogen-substituted bisimidazole photoinitiators. While these photoresists are very light-sensitive, the addition of bisimidazole photoinitiators has disadvantages. For instance, the addition of bisimidazole compounds can lead to unwanted, premature polyimide formation in solution, and thereby considerably reduce the stability of the photoresist solution. Furthermore, the halogen substituents can be split off, for example in the course of subsequent hardening of the relief structures at elevated temperatures, and can in turn have an adverse effect on the materials, in particular on the substrates. Moreover, even radiation-sensitive acrylate compounds, in particular those disclosed in European Patent A No. 47,184, tend to polymerize prematurely, which likewise markedly affects the stability of such photoresist solutions.
German Patent Application No. 3,227,584, corresponding to U.S. patent application Ser. No. 516,399, filed July 22, 1983, discloses improved photoresist prepolymers comprising partially etherified alcohols bonded through ester linkages and etherified with allyl alcohol and/or allylthiol.
A need therefore continues to exist for ways to modify these photoresists such that sheets or films prepared therewith have an adequate degree of crosslinking after much shorter irradiation times without, however, adversely affecting the stability of the photoresist solutions, subsequent processing of the photoresists or the properties of the resulting products.
The disclosures of German Patent Nos. 2,308,830, 2,437,348, 2,437,368 and 2,437,422 and German Patent Application 3,227,584, corresponding to U.S. Pat. Nos. 3,957,512, Re. 30,186, 4,045,223 and 4,088,489 and U.S. patent application Ser. No. 516,399, filed July 22, 1983 by the same inventive entity as the present application, respectively, are incorporated herein by reference.